This invention relates to a method of containers covered with a protective sheet. Ordinarily, the glass bottles and other containers containing soft drinks are covered with a heat shrinkable sheet made of oriented film of polyvinyl chloride or of polystyrene and others so as to prevent breakage by external impact and to avoid scattering of broken pieces.
Though effective to prevent scattering of fragments of broken glass bottles, such heat shrinkable sheets do not prevent breakage of the glass bottles because the cushion effect is rather low.
It has been tried, therefore, to improve the cushion effect of heat shrinkable sheets or to provide a cushion layer between a heat shrinkable sheet and the glass bottle.
In the former case, a foamed layer is provided on the surface of the heat shrinkable sheet, the glass bottle is telescopically encircled by this heat shrinkable sheet with the foamed layer, which is then shrunken by heat treatment.
In the latter case, a low-temperature heating type foaming ink is applied to the back side of a heat shrinkable sheet to be dried, glass bottles are telescopically encircled with the heat shrinkable sheet keeping the ink covered side onto the glass bottles, then the heat shrinkable sheet is shrunken by heat treatment and the foaming ink is expanded at the same time to provide a foamed layer.
In the former case, however, the sheet is broken if a scratch is made on the surface since the foamed layer is provided on the outside of the heat shrinkable sheet. In the latter case, no such problem occurs because the foamed layer is provided on the inside of the heat shrinkable sheet. However, waviness occurs on the outer surface of the heat shrinkable sheet as it is difficult to adjust the degree of shrinking of the heat shrinkable sheet and the degree of foaming of the foaming ink (i.e., the degree of foaming gas generation) at the time of heat treatment.
To provide a foamed layer by expanding the foaming ink, it is necessary to expand the foaming ink by overcoming the thermal contraction of the heat shrinkable sheet, which requires a large quantity of gas generating type foaming agent in the foaming ink. Increasing the quantity of the foaming agent, however, makes it more difficult to balance the thermal contraction of the heat shrinkable sheet and the foaming power of the foaming agent. This results in the formation of projected layers in the form of lateral strips on the surface of the sheet after heat shrinking, and the appearance is degraded significantly.
This phenomenon occurs not only in the case where a low temperature heating type foaming ink is applied to the back of heat shrinkable sheets, but also when a low temperature heating type foaming ink is applied directly around the surface of a bottle to be dried, and is covered with a heat shrinkable sheet for heat treatment.